For measurements in the field of optical metrology, wavelength-stabilized gas lasers (HeNe lasers) are often used as a light source. These substantially have high wavelength stability (depending on the stabilization process) and a long coherence length of several hundred meters. As a result, these beam sources are particularly suitable for use as frequency and wavelength standards and enable great measurement ranges for interferometric measurement systems. Typical applications comprise e.g. linear interferometers, wavelength standards, vibrometers and the use as an interferometer light source in a laser tracker.
However, a disadvantage of the use of gas laser sources (HeNe laser light sources), in particular of laser trackers, in view of generally sought after miniaturization is the size thereof, which determines the radiant flux. The power of the light source in this case depends significantly on the length of the laser tube, i.e. the achievable emission power increases with increasing length of tube. Moreover, such a laser source usually exhibits a relatively high power dissipation. A further disadvantage is constituted by the high-voltage supply required for operation. By way of example, a voltage of approximately 7000 V must be provided for igniting the laser and a voltage of approximately 1500 V must be provided during operation, as a result of which special components (e.g. high-voltage power supply and shielding) have to be used and safety measures have to be taken when using such light sources. The sensitivity in relation to magnetic fields (e.g. generated by internal motors or external welding transformers) and the restricted life of the tubes (typically approximately 15,000 operating hours) are also disadvantageous when using HeNe lasers—for example because the light sources in the systems often need to be replaced at great cost.
In this context, e.g. laser diodes are alternative light sources. These are, per se, compact, cost-effective and have a low power consumption. However, conventional Fabry-Pérot laser diodes are not suitable as interferometer light sources since these have comparatively short coherence length and do not emit in (longitudinal) single mode fashion (i.e. they emit with a plurality of wavelengths).
However, for example, the following can be used as beam sources:                distributed feedback lasers (DFBs) (with a periodically structured active medium, e.g. grating),        distributed Bragg reflector lasers (DBRs) (with an optical grating outside of the active medium but disposed on a common chip),        fiber Bragg grating lasers (FBGs) (substantially like a DFB laser, but with a grating in an external fiber),        external cavity diode lasers (ECDLs) (stabilization of the laser diode by means of an external highly stable cavity, e.g. with a holographic grating),        diode pumped solid state lasers (DPSSs),        discrete mode lasers (DMDs) and/or        microchip lasers.        
Here, the beam sources are embodied in such a way that the emitted laser beam is in single mode fashion in respect of the wavelength, with a coherence length of the order of several 10 m (or a line width <1 MHz).
A stabilization to a known wavelength is additionally required for the use of such laser diodes as an interferometer light source or as a wavelength standard. By way of example, this can be brought about spectroscopically to an absorption line of an absorption medium (e.g. by using a gas cell). Here, very many absorption lines may occur in a desired wavelength range, depending on the employed absorption medium. On the one hand, so many absorption lines are then available that an absorption line for stabilization purposes can always be achieved, even in the case of manufacturing-caused scattering of the emission wavelength of the laser diode; on the other hand, this line also needs to be unambiguously identified every time the light source is started up in order to determine the emission wavelength.
To this end, it is possible, in principle, simply to stabilize to any line and identify the latter during production using an external wavelength measuring apparatus. If the diode parameters, such as e.g. temperature and current, set for this are stored and re-established during the next switch-on, it should once again be possible to land at the original line in the case of perfect actuation electronics and retrieve this using a short wavelength scan. A possible change in the setting parameters of the diode as a result of aging can be trapped by storing the respective last values. However, the method places high demands on the quality of the actuation electronics and is very susceptible in the case of possible small wavelength distances between the absorption lines.
The requirements on the measuring apparatus can analogously be transferred to measurement devices which comprise an interferometer unit for determining changes in the distance. Here, measurement devices which are embodied for continuous tracking of a target point and determining the position of this point coordinatively may generally be subsumed by the term laser tracker. A target point can in this case be represented by a retroreflector unit (e.g. a cube prism) which is targeted by an optical measurement beam of the measurement device, in particular by a laser beam. The laser beam is reflected back to the measurement device in parallel, with the reflected beam being acquired by an acquisition unit of the device. In the process, an emission or reception direction of the beam is established, for example by means of sensors for an angle measurement, which are assigned to a deflection mirror or a targeting unit of the system. Moreover, a distance between the measuring apparatus and the target point is established, e.g. by means of time of flight or phase difference measurement, in addition to the acquisition of the beam.
Laser trackers according to the prior art can additionally be embodied with an optical image acquisition unit with a two dimensional, light-sensitive array, e.g. a CCD or CID camera (CCD=charge coupled device; CID=charge injection device) or a camera based on a CMOS array, or with a pixel array sensor and with an image processing unit. Here, the laser tracker and the camera are, in particular, assembled on one another in such a way that the positions thereof relative to one another cannot be modified. By way of example, the camera can be rotated together with the laser tracker about the substantially perpendicular axis thereof, but it can be pivoted up-and-down independently of the laser tracker and is therefore, in particular, disposed separately from the optical unit of the laser beam. In particular, the camera may comprise a fisheye optical unit and pivoting of the camera may be avoided or at least only required to restricted extent due to a very large image acquisition region of the camera. Furthermore, it is possible that the camera—e.g. depending on the respective application—is embodied to be pivotable about one axis only. In alternative embodiments, the camera can, in an integrated design, be installed together with the laser optical unit in a common housing.
By acquiring and evaluating an image—by means of the image acquisition and image processing unit—of a so-called auxiliary measuring instrument with markings, the relative positions of which with respect to one another are known, it is possible to deduce an orientation of the instrument and of an object disposed on the auxiliary measuring instrument (e.g. a probe) in space. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the position and orientation of the object in space, in absolute terms and/or relative to the laser tracker.
Such auxiliary measuring instruments can be embodied by so-called sensing tools, which are positioned with the contact point thereof on a point of the target object. The sensing tool has markings, e.g. light points, and a reflector, which represents a target point on the sensing tool and can be targeted by the laser beam of the tracker, wherein the positions of the markings and of the reflector relative to the contact point of the sensing tool are known precisely. The auxiliary measuring instrument can also be, in a manner known to a person skilled in the art, a, for example handheld, scanner equipped for distance measurement for contactless surface measurements, the direction and position of the scanner measurement beam used for the distance measurement relative to the light points and reflectors arranged on the scanner being known precisely. A scanner of this type is described in EP 0 553 266, for example.
Furthermore, in modern tracker systems—increasingly in a standardized manner—a sensor (PSD) is used to establish a deviation in the received measurement beam from a zero position. In this context, a PSD is intended to be understood to mean an area sensor that operates locally in the analog domain and that can be used to determine a focus for a light distribution on the sensor area. In this case, the output signal from the sensor is produced by means of one or more photosensitive areas and is dependent on the respective position of the light focus. Downstream or integrated electronics can be used to evaluate the output signal and to establish the focus. In this case, the position of the focus of the impinging light point can be ascertained very quickly (microsecond range) and with a nanometer resolution.
This PSD can be used to determine a deviation in the impingement point of the acquired beam from a servo control zero point, and the deviation can be taken as a basis for readjusting the laser beam to the target. For this purpose and in order to achieve a high level of precision, the field of view of this PSD is chosen to be comparatively small, i.e. to correspond to the beam diameter of the measurement laser beam. Acquisition using the PSD takes place coaxially with respect to the measurement axis, as a result of which the acquisition direction of the PSD corresponds to the measurement direction.
For distance measurement, laser trackers in the prior art have at least one distance measurement device, said distance measurement device possibly being in the form of an interferometer, for example. Since such distance rangefinders can measure only relative changes in the distance, what are known as absolute distance measurement devices are installed in today's laser trackers in addition to interferometers. By way of example, such a combination of measuring means for distance determination is known by means of the product AT901 from Leica Geosystems AG.
The interferometers used for the distance measurement in this connection can—on account of the large coherence length and the measurement range permitted thereby—comprise HeNe gas lasers or the abovementioned laser diodes as light sources, which laser diodes have stated advantages in terms of power consumption and space requirement. A combination of an absolute distance measurement device and an interferometer for determining distance with an HeNe laser is known from WO 2007/079600 A1, for example. Use of a laser diode as an interferometer laser light source is described, for example, in European patent application no. 11187614.0.
For the purposes of a reliable distance measurement or a measurement of the change in distance in the case of using a laser diode, which is to be sought-after in view of the aforementioned advantages, the wavelength of the employed measurement radiation must be stabilized and known precisely in the process. Here, as mentioned previously, high demands are placed on the quality of the actuation electronics, with the latter being very susceptible in the case of possibly small distances between the wavelengths of the absorption lines. Therefore, the re-set wavelength cannot be generated with absolute reliability, despite reproducing known actuation parameters and a stabilized wavelength generated thereby. Particularly if e.g. two or more absorption lines lie close together with a small line distance therebetween and the laser diode, when it is started up, is stabilized using one of these lines, but the current line used for stabilization cannot be determined with certainty, a precise determination of the emission wavelength is impossible.